In the past, there has been proposed an image monitoring device which controls an air conditioner and a lighting according to the number of persons who pass a pathway (e.g., see document 1 [JP 2006-270865 A]). This image monitoring device includes multiple (plural) wide angle cameras, a controller, and a monitor. The wide angle cameras are located on respective particular regions in the pathway. The controller determines the number of persons for each region in the pathway by use of images sent from the wide angle cameras, and controls the air conditioner and an output of the lighting according to an average of these numbers. With using this image monitoring device, it is possible to control an environment (e.g., an air conditioning environment and a lighting environment) in accordance with the number of persons in the pathway.
Further, there has been proposed a lighting system employing an image sensor. FIG. 6 shows a schematic diagram illustrating an example of this lighting system. This lighting system includes an image sensor 10 configured to take an image of a predetermined area and plural lighting fixtures 20 configured to control light outputs according to a detection result from the image sensor 10 respectively. The image sensor 10 and each lighting fixture 20 are connected through the transmission line L2.
The image sensor 10 detects reflected light derived from light emitted from the lighting fixture 20 by use of light sensing devices arranged in a two-dimensional manner, and creates a two-dimensional image by means of converting the intensity of detected light into a brightness data. In this prior art, the image sensor 10 creates plural temporally sequential images, and a variance of the brightness value of each pixel in the images from past to present to create a background image, for example.
Further, the image sensor 10 determines a difference between the brightness values of the created background image and a present image, and creates a background difference image constituted by such differences which are not less than a predetermined threshold.
Furthermore, the image sensor 10 calculates a difference between the brightness values of a present image and a previous image one frame ago, and creates an interframe difference image constituted by such differences which are not less than a predetermined threshold.
The image sensor 10 detects a person based on the differences present in the aforementioned background difference image or the interframe difference image, and selects a region in which a person is detected, as a detection region, and lights, at a dimming level, the lighting fixture 20 which is located in a vicinity of the detection region.
Note that, an area indicated by broken lines “a1” in FIG. 6 represents a sensing area of the image sensor 10, and an area indicated by solid lines “a2” represents a lighting area of each lighting fixture 20.
In the aforementioned lighting system shown in FIG. 6, to detect a person, the background difference image and/or the interframe difference image is used. However, when a person in the imaging area keeps its posture (e.g., a standing posture and a sitting posture), the background image including such a person is created. In this case, such a person may not be detected on the background difference image and/or the interframe difference image.
In view of the above, to detect a slight movement in such a situation, there has been proposed a method of increasing detection sensitivity by decreasing the threshold for determining the difference. However, when the threshold is decreased, a change in the light output of the lighting fixture 20 is likely to be determined as the difference.
Further, the image sensor 10 designed to detect reflected light has relatively low detection performance while the dimming rate for the lighting fixture 20 is relatively low. For example, when the dimming rate for the lighting fixture 20 is decreased down to a lower limit due to misrecognition that a person is not present, detection of a person becomes difficult as well as visibility for a person is decreased, and this may cause a loss in comfort.